There are many ways you can choose to mount
your motor-wheel assemblies to the platform.

The important thing is to mount them in
parallel so that the two wheels are pointed in the same direction. Also,
they should be the same distance forward and aft (specifically, both right in
the middle). And the tires should have plenty of clearance in the wheel
cutout holes in the platform.

Usually, the motors are mounted on the bottom
side of the platform. This will give you a platform height of about 2 1/4
inches. If, for some reason, you want the platform lower, you can mount
the motors to the top surface which will give a platform about 1 1/4 inches
high.

A number of different mounting techniques are
shown below. You can make your own out of aluminum or brass bar or
angle. You can buy a servo mounting box or mounting brackets from a hobby
store. You can mount the servo under a retaining plate. This has the
advantage of making the servo angle and position easy to change. Or,
if you're really lazy, like me, you can use foam double sided tape to hold the
motors in place. But, make sure you get them on straight. The
position is difficult to change.